This invention relates to a ball screw and nut mechanism which includes an elongated screw loosely received within the bore of a nut and opposing helical grooves are formed on the screw and in the bore of the nut. A train of balls travels in one or more convolutions of the grooves which constitute the active portion of a closed circuit. The latter is completed by a return tube which is on the nut and which defines the inactive portion of the circuit. The lead of the groove on the nut is shifted slightly from the lead of the groove on the screw so that the mechanism is preloaded.